Among advanced materials, titanium oxide gets attention increasingly because it shows applied properties unlike conventional white pigment and materials having high refractive indices (for example, refer to non-patent document No. 1). For example, in photocatalyst or separating function of electron charge of pigment sensitization, titanium oxide is an outstanding catalyst material, and has a big hope for applied field of device for a next-generation photocatalyst and a solar cell. Also in fuel cell which gains considerable attention in energy problems, research and developments of electrode layers in which platinum nano-particles are buried in titanium oxide is in the limelight. Hydrogen production is required for practical implementation of a fuel cell, and among catalysts for hydrogen production, a composite of titanium oxide and platinum is an effective candidate.
Recently, in titanium oxide, photocatalytic function which decomposes spontaneously stain and harmful materials, etc., and makes them harmless, gets attention extremely. The applied field spreads to housing, automobiles, medical treatment, soil treatment, etc., and the photocatalytic function is ranked as an indispensable technique for constructing a recycling-oriented society. However, in order to make reveal the photocatalytic function of titanium oxide, it is a precondition to use ultraviolet ray as a light source. Taking into account that natural light (sunray) contains only ultraviolet of about 3% and most of the light is visible light, use of sunray with photocatalyst which absorbs ultraviolet only becomes almost meaningless. That is, in order to use titanium oxide as an effective photocatalyst, it is most important to convert bounds for absorption of titanium oxide itself to visible light. Therefore, in many researches, atomic doping into titanium oxide which may be said as impurities, for example, anion (nitrogen atoms, carbon atoms, sulfur atoms, phosphorous atoms) doping, or cation (metal ions such as transition metals, rare earth metals, semimetals and so on) doping have been investigated.
Several typical methods is known for doping to titanium oxide. For example, in case of preparing titanium oxide by a sol-gel reaction of titanium compounds, such method is cited in which metal ion compounds are added simultaneously and colored titanium oxide is produced by burning obtained hydrolysate with heat (for example, refer to non-patent documents from No. 2 to No. 4). As such method is the most simple sol-gel reaction, the method has been used easily by many researchers, however, different results are apt to be obtained on experimental conditions, and so it is recognized that reproducibility is almost never assured. Namely, a production method of mixing metal ion compounds with simple sol-gel reaction, high-grade reaction of doping is impossible to control. Also, it is impossible to analyze what kind of material is obtained (a percentage of a constituent, a location position, kinds of different doped atoms). Therefore, the photocatalytic reaction was not reproducible, and the method was hardly to be said to be a production method of doped titanium oxide utilized for industrial use.
In order to escape from the situation, to use ion-implantation method by acceleration voltage is developed (for example, refer to patent document No. 1). In the patent document No. 1, metal ions are doped to a crystal plane of titanium oxide with implanting metal ions to crystal plane with accelerated energy. The method is certainly effective as the method of controlling doping reaction with fitting purposes because the method is capable of controlling the quantity and kinds of implanting ions. Moreover, it is confirmed that the titanium oxide obtained by the method shows absorption in the visible light region from 400 to 500 nm, and that the titanium oxide obtained by the method functions as visible light-responsive photocatalyst.
Also, doping with atoms which have comparatively high impurity level such as iron and aluminum in crystal lattice of titanium oxide, by using ultrahigh-temperature gas plasma method (for example, refer to patent document No. 2). However, the production cost of the ion-implantation method with accelerated energy, and the ultrahigh-temperature gas plasma method is high with the process themselves, and it is not too much to say that both methods are absolutely unsuitable for supplying visible light-responsive photocatalyst to a recycling-oriented society.
Also, titanium oxide doped with nitrogen atoms gets many attentions for several years as having visible light-responsive photocatalyst function. For example, production of yellow-colored titanium oxide powder is proposed by titration of ammonia water into hydrolytic reaction of titanium tetrachloride, then burning with heat the obtained hydrolysate (for example, refer to patent document No. 3). In the patent document No. 3, it is described that a signal which is considered to be derived from radical, under irradiation of visible light, in a spectrum of electron-spin resonance (hereinafter, written for short as ESR) measured at room temperature, is observed with g value between 1.930 and 2.030.
In addition, it is also reported that a visible light-responsive photocatalyst containing titanium oxide doped with carbon atoms is obtained by hydrolyzing titanium tetrachloride in the presence of tetrabutyl ammonium hydroxide, then by treating the obtained precipitate with heat (for example, refer to non-patent document No. 5).
Still more, it is also reported that a signal with g value of 2.03 which is a photocatalyst having carbonic precipitate on the surface of titanium oxide and is attributed to the carbonic precipitate or sodium in no presence of light, in ESR spectrum measured at room temperature (for example, refer to patent document No. 4).
However, because titanium oxide obtained in the patent document No. 3 and in the non-patent document No. 5 has yellow color, so it can only utilize effectively visible light of at most 600 nm though it states visible light-responsive, and the function as visible light-responsive photocatalyst is low. Moreover, when the measurement of ESR is performed in the darkness, the signal is not substantially observed. Therefore, the application had its limitation where sufficient visible light could not be obtained. Also, the production method of titanium oxide proposed in the non-patent document No. 5 is a method of burning precipitate which is produced from a mixed system of strong basic compounds like quarternary hydroxyl ammonium and titanium tetrachloride. There is no essential difference between the method and the conventional burning method with heat of a precipitate produced from a sol-gel reaction of alkoxytitanium, and the reproducibility is poor as described above. In addition, detailed investigation into the bond state concerning doped carbon atoms is not performed. Namely, structural analysis of doped titanium oxide is not performed, and it is discussed that the photocatalytic function of visible light depending on a percentage of carbon is only paid attention. Also, with a photocatalyst proposed in the patent document No. 4, a peak derived from sodium is observed which is not used clearly as source material, and also the quantity of existence is measured, so it is difficult to say that the structure of the obtained titanium oxide is controlled. The production method is based on the above-described plasma method, so in view of looking at the production method from the point of view of reproducibility, industrial production method, the method cannot be expected to practically use.
The structure and properties of industrially useful titanium oxide as visible light-responsive photocatalyst are not yet sufficiently specified, various parameters are used by researchers, and are not standardized. Therefore, a production method of doped titanium oxide which is both reproducible and simple, establishment of an identification method (analytical means) with the structure of doped titanium oxide, furthermore connection of the structure of obtained doped titanium oxide with the function as photocatalyst, etc. are the most important technical problem at present.